I am not clear on the order in which Yang Lu chan and Wu Yu Xiang collaborated on their respective arts.

who found the "taiji classics" of Wang Zongyue in the salt shop?

not sure if this is right-

Yang lu chan was working at a medicine shop in Yongnian and heard of chen village taiji and went there to train in Laojia (old frame). So he came back to Yongnian many years later and heard of Wu Yu xiang and taught him some taijiquan. Wu Yu Xiang later went to chen village to learn more and learned Xiao jia- (new frame- small style)created his style and taught it to Li yi yu.

so when did Wu Yu Xiang send Yang Lu Chan to Beijing to teach there?

I am more curious Yang lu chan and Wu yu Xiang's friendship/collabroration in the entire early process of taijiquan going public and why their taijiquan is completely different even though they both went to chen village.

It is stated that Wu and Yang shared their taijiquan experiences up to a point. Wu thought Yang may have been holding back so he went to Chen Qingping (I think) to learn the essence (stuff he though Yang was not teaching) of taijiquan from the source.

Keeping in mind Zhaobao and Jiagou(chen) had different training regimen and understanding.

What do you mean by different since the esence is the same? Same art but as usually different physical expression, each person showing that differently based on posture, training and degree of athleticity.